#21 on Practice Final

[Abigail Urbina 1K]

For those of you who went to the review session last night to go over the questions on the practice final, could you explain #21 to me? We are being asked to find the oxidation state of iron in the heme complex. In order to find it, we must find the formal charges of the nitrogen atoms. He demonstrated that 2 are neutral and the other 2 have a -1 charge.

Could someone please explain to me how he came up with those charges? I can't seem to figure out how he got those formal charges just by looking at the diagram.

[Chem_Mod]

When you determine oxidation states, you look at all the bound atoms as if they were not bound. So instead of having each nitrogen single bonded to the Fe, treat each bond as a lone pair on the nitrogen. Then work out the formal charges. Keep in mind that 2 of the nitrogens already have one lone pair to begin with that is not shown. So when the bond is treated as a lone pair, the nitrogen then has 2 lone pairs and 2 bonds, which gives it a -1 formal charge.

-L

[Cam Bear 2F]

In what instances do you just treat the bond as a lone pair?